In order to evacuate an enclosure to a desired low pressure of gas, it is known to provide the enclosure with a hollow tubular member through which the gas is evacuated. After the enclosure has been evacuated to a desired gas pressure, the tip of the tube is crimp sealed under pressure to prevent the entry of external gases into the vacuum enclosure. Typically the tubular member, or tip-off tube, is comprised of copper. During the crimping process a self-welding occurs within the tube, this self-weld providing for hermetically sealing the tube.
It has been found, however, that a number of problems are inherent in the use of such copper tip-off tubes. One problem relates to the relatively small surface area within the tube which comprises the self-weld. As can be appreciated, the greater the surface area which comprises the weld, the greater will be the overall strength and resistance to mechanical loads within this area. This increase in strength is directly related to the reliability and durability of the self-weld and, hence, to the overall reliability and durability of the vacuum enclosure itself.
Another problem related to the use of copper tip-off tubes is the need to remove, before crimping, a layer of copper oxide which is typically found within the tube. This oxide removal is required to insure the quality of the self-weld during the crimping process. This oxide removal is typically accomplished by cleaning the tube with a suitable acid or chemical solvent and possibly also abrasively cleaning the inside of the tube. Not only does this cleaning introduce additional processing steps in the manufacture of the vacuum enclosure, with a consequent increase in manufacturing costs, but traces of the cleaning acid or solvent may remain within the tip-off tube. After crimping the tube, these residual contaminates which are locked in the pinch-off area may diffuse into the vacuum enclosure itself. Inasmuch as vacuum enclosures typically include a "getter" to absorb trace amounts of atmospheric gases and molecular species which may outgas from assemblies within the enclosure, these additional contaminants may over-load the getter, resulting in the failure of the getter.